Field of the Invention
The invention relates in general to multimedia signal processing technologies, and more particularly to encoding/decoding technologies in video systems.
Description of the Related Art
Digital television broadcasting has matured and become popular with the ever-improving communication technologies. In addition to being transmitted through cables, digital television signals may be propagated in form of wireless signals via base stations or artificial satellites. To satisfy demands on enhanced image quality and reduced transmission data amount, a transmitter end usually encodes and decompresses audio/video signals to be transmitted. Correspondingly, a receiver end needs to correctly decode and decompress the received signals in order to restore the audio/video signals.
FIG. 1 shows a partial functional block diagram of an encoding system compliant to the digital audio video standard (AVS). An intra-prediction module 12 performs an intra-prediction process on image blocks in a video frame to generate luminance residual blocks corresponding to the image blocks. The luminance residual block is provided to a discrete cosine transform (DCT) module 14 for a DCT process to generate a DCT coefficient matrix. To further reduce the data amount, a secondary transformation module 16 performs a secondary transform on low-frequency components in the DCT coefficient matrix. For the AVS encoding system, regardless of the size of the DCT coefficient matrix, the secondary transformation module 16 performs the secondary transform on only 4*4 low-frequency components at the upper left corner. The low-frequency components having undergone the secondary transform and other high-frequency DCT coefficients that have not been processed by the secondary transform are recombined at a quantization module 18, which then performs a quantization process.
In practice, the DCT process the DCT module 14 performs includes a set of DCT performed along the vertical direction and a set of DCT performed along the horizontal direction. Similarly, the secondary transform the secondary transformation module 16 performs is jointly formed by a set of secondary transform performed along the vertical direction and a set of secondary transform performed along the horizontal direction. According to AVS specifications, the DCT module 14 is required to first perform the DCT process on the luminance residual blocks one row after another along the horizontal direction till all the DCT process along the horizontal direction is complete, and then to perform the DCT process one column after another along the vertical direction. Further, according to AVS specifications, the secondary transformation module 16 is required to first perform the secondary transform on the low-frequency components in the DCT coefficient matrix one column after another along the vertical direction till all the secondary transform along the vertical direction is complete, and then to perform the secondary transform one row after another along the horizontal direction. FIG. 2(A) illustrates a sequence relationship of the above transformation processes.
Take an example where the size of a luminance residual block is 4*4. FIG. 2(B) shows a detailed timing relationship between typical DCT process and secondary transform in an AVS encoding system. Symbols R0 to R3 represent four row in a block, and symbols C0 to C3 represent four columns in the block. As shown in FIG. 2(B), in work cycles 0 to 3, the DCT process performed along the horizontal direction is sequentially performed on the rows R0 to R3; in work cycles 4 to 7, the DCT process performed along the vertical direction is sequentially performed on the columns C0 to C3. After the DCT process performed on the column C0 is complete, contents of the column C0 are no longer affected and changed by the DCT process, and so the secondary transformation module 16 may start performing the secondary transform on the column C0 along the vertical direction from the work cycle 5, and so forth. In work cycles 6 to 8, the secondary transform performed along the vertical direction is sequentially performed on the columns C1 to C3; in work cycles 9 to 12, the secondary transform performed along the horizontal direction is sequentially performed on the rows R0 to R1. As seen from FIG. 2(B), a period of 13 work cycles is needed for completing the DCT process and the secondary transform for a 4*4 luminance residual block. In comparison, if the secondary transform is not performed, a period of only 8 work cycles is needed for completing the DCT process for a 4*4 luminance residual block. That is to say, in a typical AVS encoding system, although the secondary transform contributes the benefit of a reduced data amount for encoded results, the encoding process is lengthened and thus overall system performance is degraded.